A method to compute the interfacial excess free energy of systems where a liquid phase is interacting with a solid phase is presented. The calculations are carried out by means of molecular dynamics simulations. The algorithm is based on a thermodynamic integration scheme that reversibly turns a flexible atomistically detailed solid surface that interacts with a liquid phase into a flat surface and allows the calculation of the variation in Gibbs free energy. The approach is probed by applying it to a model system of Lennard-Jones particles and comparing to previous calculations on similar systems.